Natural-abundance 13C NMR spectroscopy yields resolved resonances of individual nonprotonated aromatic carbons of small native proteins in solution. The observed single-carbon resonances are used to study: (1) Properties of chemically modified proteins. (2) Titration behavior of tyrosine and histidine residues. (3) Motions of aromatic side-chains. (4) Conformational changes. (5) Self-association. (6) Environment of the copper in "blue" copper proteins. (7) Structure of the carbohydrate portions of glycoproteins.